What were the most powerful kingdoms to develop on mainland southeast asia by ad 1500

The Khmer Empire.

Explanation:

The Khmer empire is one of the ancient kingdoms of south-east Asia in the 6th century, the Khmer migrated from India and established their empire which today is known as the Cambodia and Laos.The Khmer empire capital was established in the area of Angkor.The Khmer empire was largely influenced by the Hindu culture most of them are Hindus and they worshiped Lord Shiva and other Hindu gods.The greatest achievement of the Khmer empire was in the field of sculptures and architecture.The Angkor period was the golden age of the Khmer civilization saw the greatest achievements of the empire. Thai captured Angkor in 1434 led to the end of the great Khmer civilization.